This invention relates to a device and a method for thermally treating semiconductor wafers having at least one silicon layer to be oxidized and a metal layer, preferably a tungsten layer, which is not to be oxidized.
In the semiconductor industry there is increasingly the necessity to design semiconductor elements which are smaller and more efficient. An example of this is the increasing use of tungsten instead of tungsten silicide as a component part of the gate assembly in MOS transistors. The use of tungsten as the gate material is advantageous due to the low specific resistance of tungsten compared to tungsten silicide. This makes it possible to reduce the resistance of the gate, and this leads to improved electrical performance of the transistors. Moreover, the height of a gate stack can be considerably reduced by the use of tungsten in contrast to tungsten silicide, and this leads to a substantial simplification in the following filling and etch steps because the aspect ratio (ratio of height to the distance of the stack) is smaller.
A gate stack of this type is generally formed on a silicon substrate, whereby first of all a gate oxide layer, a polycrystalline silicon layer, a tungsten nitride layer, a tungsten layer and a silicon nitride layer are applied.
Subsequently, for defining individual gate areas, selective etch is carried out, whereby the cauterized side walls of the gate stack are open. After the etch there follows a gate stack side wall oxidation which serves to mend or heal etch damage and reduce the leakage currents. When using a tungsten gate, this process must be selective, i.e. whereas the polycrystalline silicon layer has to be oxidized, the tungsten must not be oxidized because the formation of a high-resistance tungsten oxide prevents the electrical functionality of the gate.
Selective oxidation of this type can be achieved by wet oxidation in hydrogen-rich atmospheres within a fast heating unit or a rapid thermal processing (RTP) unit.
An RTP unit which can generally be used here is, for example, shown in DE 44 37 361 going back to the applicant.
With the known RTP unit, the wafer to be treated is received in a quartz chamber and is heated by banks of lamps located above and below the quartz chamber. In so doing, it is known to place a thermally stable and geometrically similar light-absorbing plate in the quartz chamber, and in such a way that the wafer is heated by a radiative and convective energy coupling between the plate and the wafer, instead of being heated directly by the lamps. The advantage of this is that the light-absorbing plate is thermally stable and maintains constant emissivity, by means of which exact temperature control of the semiconductor wafer is possible, even if the emissivity of the semiconductor disc is varied. For details with regard to the use of a light-absorbing plate, reference is made to DE 44 37 361 so as to avoid repetitions.
With the process described above for the selective side wall oxidation, there is, however, the problem that compounds containing tungsten, in particular tungsten oxide, can evaporate from the semiconductor wafer. This type of evaporation of the tungsten oxide or of a metal oxide or metal hydroxide is deposited on the quartz chamber, is reduced there to tungsten or to metal and by partial shading of the radiation from the lamps, affects the temperature distribution over the wafer and/or over the light-absorbing plate and so also over the wafer. With several processes following on from one another, due to stronger and stronger shading processes, substantial changes can be made to the process parameters, such as e.g. the process temperature and the wafer temperature.
The aim of this invention, therefore, is to create a device and a method for thermally treating semiconductor wafers, whereby the process conditions which substantially remain the same, make it possible with selective oxidation of a silicon layer to be oxidized and of a metal layer, preferably a tungsten layer, not to be oxidized.